It is generally known that many dry granular materials do not flow by themselves and that agitation or vibration is necessary to keep the granular materials flowing through the hopper. Vibrators of the type contemplated herein are shown in U.S. Pat. No. 1,548,385, issued on Aug. 4, 1925, to W. E. Prescott, and entitled "Feed Hopper;" U.S. Pat. No. 2,560,480, issued on Jul. 10, 1951, to W. B. Rogers, et al., and entitled "Hopper Shaker Plate And Comb Assembly;" and Russian Patent No. 1381037, issued on Mar. 15, 1988, to G. G. Zurabishvili, and entitled "Granulated Material Bunker." Vibration of hoppers of this type often cause segregation of the different size materials being fed wherein the heavier, denser materials settle in the hopper and are fed into the feed screw before the lighter material.
Internal stirring devices have also been used to keep materials from bridging over the feed screw at the bottom of the hopper. If the materials fail to flow into the flights of the feed screw an interrupter is used to break up the material on the top of the feed screw. The internal stirring devices also change the density of the material by pushing the material in the front of the stirring device until it is compacted. As the stirring device rotates around the hopper the more dense material is pushed into the flights of the feed screw. As these flights turn the next couple of flights receive the lighter material that follows immediately after the stirring device. This resulted in erratic feeding of the material making it hard to maintain feed accuracy.
In order to obtain accuracy in feeding dry materials from a volumetric screw feeder two things must be considered. First, that the flights of the screw are completely filled with a uniform density material. Second, the screw had to be turned at a constant rate with enough of a turn down ratio to be able to key in on the rate of flow one desired.
Flexible vinyl hoppers are now available which are designed for mass flow so that material "first in" is also "first out." The flexible hopper is massaged externally to keep the granular materials from bridging over the feed screw. One system for accomplishing this result uses two paddles which rest against the outside of the flexible hopper. The paddles are undulated in such a fashion so as to break any bridges and still keep the material at a constant density. This gentle form of agitation keeps the hopper from overworking and also keeps the material being fed from being broken up or degraded. The panels rest against the sides of the flexible hopper and can be set to move a maximum amount or just a small amount depending on the density and type of material to be fed through the hopper. If the material is heavy and could be easily segregated, the panels are set to move a small amount to barely keep the material moving through the feed screw. Light materials, on the other hand, have to have maximum movement so the effect of the paddles is transferred through the entire hopper. The flexible vinyl hopper concept with the outside paddles massaging material enables one to key in on the specific characteristics of the material being fed to provide for a higher degree of accuracy than ever before capable from a volumetric feeder. However, the continuing sliding motion of the paddles against the surfaces of the vinyl hopper tend to wear or rub away the vinyl, weakening the walls of the hopper.